In the catalytic cracking of gas oil, gasoline quality is currently controlled by primarily varying the catalyst activity and reactor operating conditions. Gasoline of relatively good octane quality, i.e. having a research clear octane rating of 91 to 93 can be made using these techniques.
The use of varying catalytic systems to improve the octane quality of the gasoline recovered from the cracking of hydrocarbons is disclosed in U.S. Pat. Nos. 3,788,977 to Dolbear et al., 3,830,725 to Dolbear et al. and 3,835,032 to Dolbear et al. These patents disclose the use of zeolite metals impregnated into or exchanged into the zeolite component of zeolite-promoted cracking catalysts to improve the octane content of the gasoline recovered from the cracking process. It is generally known that hydrocarbon cracking catalysts which are promoted with stabilized zeolites, particularly ion exchanged synthetic faujasite, are capable of producing high yields of gasoline fractions from petroleum feedstocks such as gas oil. These cracked gasoline fractions are subsequently combined with octane enhancing additives such as tetraethyl lead to produce high octane motor fuel.
Recent emphasis on air pollution control has dictated a need for removing metal-type octane enhancing additives from commercial gasolines. It is generally necessary for the refiner to use a blend of petroleum gasoline fractions which have an inherently high octane rating to produce non-lead gasoline of sufficient octane rating for use in modern automobile engines. Highly aromatic gasoline fractions are of particular use to the refiner. Unfortunately, however, the gasoline fractions produced by the catalytic cracking of gas oil using normal amorphous and crystalline zeolite type cracking catalysts are of relatively low aromatic content.
It has been reported in the literature [Van Hook, W. A. and Emmett, P. H.: Journal of Am. Chem. Society, 84, p. 4410 (1962)] that olefins react in a cracking atmosphere to polymerize, alkylate and/or cyclize to yield higher molecular weight hydrocarbons and even aromatics through a dehydrogenation step.